Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of customers, resulting in a large loss of profits. By utilizing pre-press color proofing, time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of print media by transferring dye from a sheet of dye donor material to the print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a print head, and a vacuum imaging drum), and print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the print media (in roll form) from the material assembly or carousel. The print media is then measured, cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor material (in roll form) is also metered out of the material supply assembly or carousel, measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the print media (which has already been secured to the vacuum imaging drum).
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the print media and the dye donor material on the spinning vacuum imaging drum while it is rotated past the print head that will expose the print media. The translation drive then traverses the print head and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the print media.
After the intended image has been written on the print media, the dye donor material is then removed from the vacuum imaging drum. This is done without disturbing the print media that is beneath it. The dye donor material is then transported out of the image processing apparatus by the dye donor material exit transport. Additional dye donor materials are sequentially superposed with the print media on the vacuum imaging drum, then imaged onto the print media as previously mentioned, until the intended image is completed. The completed image on the print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The scanning subsystem or write engine of the lathe bed scanning type comprises the mechanism that provides the mechanical actuators for imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the print media and the dye donor material from the vacuum imaging drum. The scanning subsystem or write engine provides the scanning function by retaining the print media and dye donor material on the rotating vacuum imaging drum, which generates a once per revolution timing signal to the data path electronics as a clock signal while the translation drive traverses the translation stage member and print head axially along the vacuum imaging drum in a coordinated motion with the vacuum imaging drum rotating past the print head. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the print media.
The translation drive permits relative movement of the print head by synchronizing the motion of the print head and stage member such that the required movement is made smoothly and evenly throughout each rotation of the drum. A clock signal generated by a drum encoder provides the necessary reference signal accurately indicating the position of the drum. This coordinated motion results in the print head tracing out a helical pattern around the periphery of the drum. The above mentioned motion is accomplished by means of a dc. servo motor and encoder which rotates a lead screw that is typically, aligned parallel with the axis of the vacuum imaging drum.
The print head is selectively locatable with respect to the translation stage member, thus it is positioned with respect to the vacuum imaging drum surface. By adjusting the distance between the print head and the vacuum imaging drum surface, as well as an angular position of the print head about its axis using adjustment screws, an accurate means of adjustment for the print head is provided.
The translation stage member and print head are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. A DC servo drive motor induces rotation to the lead screw moving the translation stage member and print head along the threaded shaft as the lead screw is rotated. This achieves a movement of the print head relative to a longitudinal axis of the vacuum imaging drum. The lateral directional movement of the print head is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. Registration of the print head, that is, positioning the print head repeatably in the precise location for the beginning of a scan, is a significant problem. Colorant transfer action prints dots (nominally 4-8 microns in diameter) on the receiver medium, with the dots positioned at a precise distance from each other (with dot centers nominally 10-12 microns apart). To maintain correct registration of dots from one color separation to the next, the print head must be precisely and repeatably positioned at identical coordinates for each pass. Relative to the imaging receiver that is secured on the drum surface, there is some tolerance for initially locating the registration position for start of scan. However, once an initial registration position is identified, the image processing apparatus requires precise repeatability, so that each subsequent registration operation brings the print head to the same fixed reference point, within very close tolerances.
Registration must be performed multiple times for each color roof, once at the beginning of each component color pass. To maximize throughput (productivity) of the device, it is advantageous to be able to perform registration as quickly as possible.
With existing color proofing systems, such as the system noted above, head registration requires a combination of high-cost components including a servo loop with an encoder, a fine-resolution lead screw, and a precision sensor to indicate linear travel. The conventional method used requires driving the translation assembly to a precise position as indicated by a linear-motion sensor, then using the servo loop to move the translation assembly back, a precise number of encoder counts, to the actual registration position.
Lead screw positioning solutions for locating a print head at a home position are well-known in the art. Among patents of particular interest that disclose various aspects and improvements on conventional head registration are the following:
U.S. Pat. No. 5,160,938 discloses a method and an apparatus for homing a precision print head relative to an imaging drum in an ink jet printer. This method locates a relative home position by using a sensor placed in the direct path of an ink jet. Repeated adjust/test cycles are used to zero in on the home position.
U.S. Pat. No. 5,074,690 discloses a head positioning and homing system for a standard impact-type printer. This method uses a timing strip built into the printer assembly itself, with a position sensor that travels with the print head carriage.
U.S. Pat. No. 4,488,051 discloses a method for homing a load element driven by a lead screw (in the preferred embodiment, this method is used in the control apparatus for positioning a diffraction grating in a spectrophotometer). Notably, this method achieves fine-tuning of the home position using a sensor for rotational position of a flag that is fixedly mounted to rotate with the lead screw.
U.S. Pat. No. 4,117,341 discloses a method for homing a lens component, driven by a lead screw, used in ophthalmic instrumentation. Here, a mechanical flag element travels with the moving lens assembly, triggering an optical sensor when the assembly reaches a reference home position.
U.S. Pat. No. 4,329,051 discloses a method for homing the position of a diffraction grating in a spectrophotometer using a control segment driven by a lead screw. Here, a mechanical stop is employed to indicate the home position of the control segment.
While the above patents disclose methods used for print head or optical component homing in a lead screw-driven device, none of these patents provide for a method or apparatus which enables the precise addressability required for registration of a print head in an imaging system that scans with a resolution at 2400 dots per inch or higher. Also, none of the above patents disclose or suggest a method or apparatus that allows straightforward adjustment of a sensor component position for optimal timing and precision.